Hot Tub on Elevated Balcony 4

[woodman1967]

Have a question about a hot tub on a balcony. Have a client who wants a two car garage with a two bedroom apt above. The garage ceiling is to be 12'. They want a balcony off the main bedroom and then place their 6 person hot tub on that balcony (12'x12'). What things should the client be warned about? That's a lot of weight on the balcony. The balcony will be engineered of course.

 

[KootK]

 

[JAE]

It depends on how large a beer that dude in the hot tub is holding.

 

[LittleInch]

Not having a support at the edge will mean very big beams if this is a cantilever.

Waterproofing the deck can be very difficult especially with a hot tub.

Not much to go on here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[woodman1967]

Thanks for your responses. Should have been clearer, the balcony will be 12'x12' and not cantilevered. Was to be a standard construction (2x10 PT joists, composite decking. Supported by a multiply 2x10 beam which was to be supported by 6x6 PT posts). The deck will be end up being 13' above grade. That was the original construction...then the client mentioned the hot tub. Articles I've read have the weight as much as 6000lbs.

 

[IRstuff]

Weight sounds plausible

https://www.costco.com/evolution-spas-highlands-75-jet,-5-or-6-person-spa.product.100730734.html

400 gal + 650 lb (dry weight) + 6*180 lb = 5068 lb

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[LittleInch]

Well design for 50psf gives you 7200lb

Very good support fixings for your beams and maybe some extra reinforcement under the tub.

But limited movement.

The devil is in the detail of the joist fixing and quality of construction.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[phamENG]

Warnings for the owner? No, not really. Other than, maybe, warning him/her that the deck has been designed for the hot tub they have selected, and they shouldn't replace it with a larger one.

They did tell you which hot tub they're buying so you can have an accurate weight, right? Don't go by articles you've read. Have them select a 'basis of design' hot tub and use that weight. Otherwise you'll either be under - and they'll mad as hell when they buy one that is 40% heavier and you have to tell them they can't install it or you need to redesign - or you'll be way conservative and they'll be paying for more deck than they need. Localized load of a hot tub is usually in the 150psf to 200psf range.

LittleInch is right - the devil is in the details. Both for you, detailing it, and for the contractor, building it. They can't just slap it up there like any ol' deck. And your design will need to pay very close attention to creep. The deck is likely to be wet. A lot. And people don't drain hot tubs very often, so that's going to be a permanent load. Long term deflections will likely control the design more than anything else. 2x12s at 12" with lots of solid blocking isn't uncommon for isolated areas just under the footprint of the hot tub. I doubt you'll get anything to work at a 12' span.

 

[lexpatrie]

You probably want to locate the hot tub limits dimensionality on the design, and if it's close to the edge there's a higher Guardrail, likely... I would not blanket design for 50 psf, place the load where the hot tub is. Or design for "excessive" 100 psf load the whole area.

As a usage note, "deck" is going to be more commonly understood as having outbound columns, "balcony" is more typically a cantilever. Hence the earlier confusion and the post title.

Warnings for the Owner? Most definitely, have the deck inspected during the worm by a competent third party, and have it inspected at the one year mark by another competent licensed engineer for signs if distress, overloading, or deterioration. That engineer can give a time frame for when the next inspection needs to happen.

I don't know these guys, but here's a nice internet article on hot tubs on decks. I think the PA in the web site means Pennsylvania.

Wellispa hot-tub on a deck,

 

[gte447f]

Blanket design the whole deck for the max load of 150-200psf. You can't control where they might put the hot tub. Cover your a$$. No warnings necessary. Just make sure they understand that the deck is being designed for 4-5 times the normal load on a residential deck, so they have to expect it to cost significantly more than the typical residential deck.

 

[lexpatrie]

Ok, since I kind of have a hot tub project, too, I'll bite.

86.6 x 86.6 footprint = 52.2 ft[sup]2[/sup] (rounded down very slightly).

and 650 lbs weight = 12.5 psf.

add 100 psf live load (occupants) = 100 psf (5,200+ pounds, that sounds like a LOT of people, not 6-8, anyway).

and 400 gallons (presuming the occupants don't cause it to overfill, using 62.5 psf for I guess salt water tub?).

400 gallons * 8.34 lbs/gallon - 3,336 lbs. (63.9 psf on the full footprint, which is unrealistic, say 3' height, then 4.2' square, so 187.5 psf, that's 34% of the footprint that seems really unrealistically small, so the load is overestimated).

1" freeboard (for overfilling) - = 86.8 x 86.8 x 1 / (12)[sup]3[/sup] = 5 psf or so.

Tub - 12.5 psf
Water = between 63.9 psf (full 52 ft[sup]2[/sup]) and 187.5 psf ( on 34% of the footprint) - 0 on the remainder.
occupants = 100 psf.

Range = 112.5+63.9 (unrealistically low) to 187.5 (unrealistically high)
Range = 176.4 psf - 300.0 psf.

So, PhamEng gets the gold star.

 

[ChorasDen]

To make your calc a little simplier, you've already identified water density at ~62.4lbs/ft^3. Just multiply by the water depth, maybe 3ft in center of tub? 62.4lbs/ft^3 * 3ft = 187.2lbs/ft^2 for the water in the tub.

 

[woodman1967]

Thanks for the help everyone.

An update on the project, the client has decided not to place the hot tub on the elevated deck. To be honest that kind of makes me nervous. Hopefully they don't plan on placing the hot tub on the deck anyway after the job is finished.

Thanks again

 

[IRstuff]

You could, and probably should, tell your client that the design will not be rated for anything like a hot tub should they, or future owners, want to add a hot tub, and possibly document that in your drawings?

I'd be concerned that if you don't do so in writing, that they might add a hot tub, have the deck collapse, and then they sue you.

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[udraft]

Should not be hard. Estimate total weight of the tub, etc. Be generous, possibly as much as x2. Install floor and support under area to support that weight. Should be able to use pounds per square foot, fairly straight forward.
Otherwise simply warn them that a balcony will not support a hot tub.

 

[lexpatrie]

That's not even a reasonable comment, sorry.

It's already been established that the load from a hot tub is quite high, (I thought the comments about 150 psf were crazy high, and I could have sworn that was from Phameng, but I don't see it above, now.) Anyway, as I posted above, it's in the realm of possible, especially if you apply 100 psf occupancy live load on it, naturally if you add 100 psf to something else you get above 100 psf.... anyway).

But.... doubling it as you suggest puts it way beyond feasible, if you know wood construction.

Designing for 340 - 600 psf is just beyond.

You could perhaps argue the 100 psf from occupant live load is too high, and on the footprint of the hot tub "assembly" live load I admit doesn't make much physical sense, but the code isn't about physical sense, the code is about how it's actually written. The actual mechanics of how that gets negotiated down to a "reasonable" level based on generally accepted principles of mechanics feels very iffy. Sure it's a six person hot tub, I suppose you could tag the thing like an elevator (that would have a life expectancy of three minutes), i.e. max 6 persons or 1,200 lbs, or whatever. (Big, Healthy, Husky, Fluffy, Dayum and Oh, Heck No).

The primary elements here are the weight of the water, water does NOT get twice as dense, even at the bottom of the Mariana trench (right? now I'm questioning that, maybe Hey Arthur isn't the best resource) https://water.lsbu.ac.uk/water/water_density.html , or at least, it doesn't get twice as dense under normal atmospheric situations, and the weight of the hot tub itself, pump and equipment (where 20% increase, maybe could be justified). Saltwater is a little denser, but not dramatically, what you're proposing is like filling the hot tub with bricks, not a popular choice, or wet concrete (120-150 pcf?) filled hot tub and those aren't popular.

 

[phamENG]

Localized load of a hot tub is usually in the 150psf to 200psf range.

Right here, @lexpatrie

 

[lexpatrie]